Maartje Boer

CHAPTER 2 32 average number of students and ethnic composition. There were no refusals on school class level, and on the individual level 92% of all selected adolescents participated. The individual non-response was mostly related to absence from school at the day of survey assessment, due to for example illness or truancy (Van Dorsselaer, 2018). Participation in the HBSC-study was voluntary and anonymous, conducted through digital self-completion questionnaires during school hours monitored by trained research-assistants. School principals sent information about the study to all parents of adolescents in the selected school classes in advance, and parents were provided the opportunity to refuse participation. Almost all parents provided this passive consent ( > 99%). Adolescents gave active consent by ticking a box at the start of the survey that confirmed their approval ( > 99%). The study was approved by the ethics council of Social Sciences of Utrecht University (FETC17-079). Measures Problematic SMU The SMD-scale was used tomeasure problematic SMU (Van den Eijnden et al., 2016). The scale consists of nine dichotomous items corresponding to the nine diagnostic criteria for internet gaming disorder as stated in the appendix of the DSM-5 (American Psychiatric Association, 2013; Lemmens et al., 2015). The questionnaire was introduced with: “We are interested in your experiences with social media. The term social media refers to social network sites (e.g., Facebook, Twitter, Instagram, Google+, Pinterest) and instant messengers (e.g., WhatsApp, Snapchat, Facebookmessenger)”. Subsequently, adolescents were asked “During the past year, have you (…)”, followed by for example “regularly found that you can't think of anything else but the moment that you will be able to use social media again?” (preoccupation). Response options were (1) yes and (0) no . The items ‘displacement’ and ‘escape’ had slightly different wordings than the initial scale (Van den Eijnden et al., 2016). Mental Health Problems Four subscales of the self-report Strength and Difficulties Questionnaire (SDQ) were used tomeasuremental health problems, including emotional problems,